1. Field of the Invention
This invention relates to an air-fuel ratio control system for use in an internal combustion engine and, more particularly, to such an air-fuel ratio control system for controlling the intake air-fuel ratio on the rich or lean side of stoichiometric.
2. Description of the Prior Art
Air-fuel ratio control systems have already been proposed which include an exhaust gas sensor such as an oxygen sensor provided in the exhaust passage of an engine. Such an oxygen sensor is responsive to the concentration of the residual oxygen of exhaust gases flowing thereover for providing an on-off type of signal around stoichiometric air-fuel conditions. The output signal of the oxygen sensor is used for feedback control of the amount of fuel supplied to the engine so as to maintain the inlet air-fuel ratio around the stoichiometric conditions.
Although such conventional air-fuel ratio control systems are convenient to be used with a three-way catalyzer having oxidation and reduction functions for totally purifying exhaust emissions, it is extremely difficult to control the inlet air-fuel ratio on the lean side of stoichiometric, for example, around 17 for higher fuel economy or on the rich side of stoichiometric, for example, around 13 for higher engine output.
This is, such an exhaust gas sensor such as a zirconia oxygen sensor, having its output sharply changed between its on and off states around stoichiometric air-fuel conditions and held substantially constant except around stoichiometric air-fuel conditions, cannot be used directly for controlling the inlet air-fuel ratio except around stoichiometric air-fuel conditions and cannot be used for controlling the inlet air-fuel ratio with great accuracy and without spoiling the performance of an engine equipped with an exhaust emission purifying system such as an exhaust gas recirculation system.